inercial

Inercial (inertial) is a term in physics relating to inertia and frames of reference. An inertial frame of reference is a coordinate system in which a freely moving body not acted on by external forces moves with constant velocity. Real frames are only approximately inertial over short times or small regions, but this idealization underpins Newton's laws.

When a frame accelerates or rotates, it becomes non-inertial, and fictitious forces such as Coriolis and centrifugal forces must be introduced. These forces are artifacts of the accelerating frame rather than real interactions.

Inertial mass measures resistance to acceleration, while gravitational mass determines gravitational interaction. Experimental tests find their ratio equal to very high precision, a cornerstone of the equivalence principle and general relativity.

Inertial concepts are used in navigation and measurement. Inertial navigation systems (INS) rely on accelerometers and gyroscopes to track position and orientation without external signals, though drift is common and is mitigated by other data. In science and engineering, inertial frames underpin classical mechanics and dynamics.

Historically, the idea arose with Galileo and Newton. In special relativity, inertial frames move at constant velocity relative to one another, and their laws hold in all such frames; general relativity extends the idea locally to curved spacetime, where gravity is geometry rather than a force.